This invention relates in general to heat transfer devices of the type known as heat pipes and more particularly to heat pipes capable of safe and efficient use under extreme operating conditions.
Heat pipes are well known and widely used in applications where it is necessary or desirable to transfer heat from a source to a receiver efficiently and with minimum temperature drop. Generally, the heat pipe is constituted of a vessel or an enclosure of selected shape containing a quantity of working fluid and a wick, usually of capillary material. Heat is applied at the evaporator portion of the vessel to vaporize the working fluid in that vicinity. The vapor is driven to the condenser portion of the vessel where it condenses, giving up heat. Aided by capillary action, the condensate then returns from the condenser portion to the evaporator portion to be vaporized again.
Heat pipes may be formed in practically any configuration and are quite versatile, especially in such applications as where it is desired to transfer heat around bends, over or under obstructions, or to concentrate or diffuse heat from a heat source at a heat receiver.
Situations have arisen, however, where it has been considered unadvisable or even dangerous to utilize heat pipes for heat transfer, even though the uniform temperature distribution and high thermal conductance of heat pipes are qualities of great value in the particular applications. For example, it is sometimes necessary to expose heat pipes to exceptionally high temperature, to high ambient pressure or to both to perform a desired transfer of heat. Other considerations have arisen where there has been reluctance to utilize heat pipes, such as in home appliances where the safety factor looms large or where the cost of materials needed in elements such as large cooking surfaces becomes important.
The present invention will be described in connection with a specific application which is illustrative of those noted above where heat pipes have previously been thought to be impractical or incapable of use.
That application is the growing of crystals of III-V compounds such as gallium arsenide or indium phosphide. These compounds are becoming of increasing importance and are replacing silicon because they are superior to silicon as semiconductors, especially in computer chip applications. Even though the improved performance is widely recognized, use of III-V compounds of the type mentioned has been limited somewhat because of the difficulty of growing these crystals as compared to silicon. Crystals of III-V compounds are grown at temperatures as high as 1600.degree. K. and pressures of the order of 1000 psi. Heat pipes capable of operation at such temperatures and pressures have been thought to be not feasible, either because of the inadequacy of the creep strength of the refractory metals from which such heat pipes are usually made or because of the fragility or expense of known highly specialized materials which have adequate creep strength. For example, tungsten, a material commonly used in high temperature heat pipes, lacks the necessary creep strength for desired thin walls for the extremes of operating conditions involved. On the other hand, silicon carbide, although it may maintain adequate creep strength under extreme conditions, is quite brittle.
One prior art technique employed in an attempt to provide heat pipes for high temperature, high pressure applications is to use chemical vapor deposition to form trilayered walls of silicon carbide-graphite-tungsten. In such structures the silicon carbide provides the necessary load-bearing strength which tungsten lacks at the high temperatures and pressures of operation. Another prior art alternative is the fabrication of heat pipe walls from relatively low strength materials but the incorporation of a large number of internal bridges or braces so that the walls will withstand the pressure of operation. Such structures, however, are exceedingly complex and costly and are not efficient in heat transfer.